BANGKOK—Thailand expanded special-security laws to cover the entire capital after protesters forced their way into government offices in a bid to oust Prime Minister Yingluck Shinawatra.

Crowds poured into government ministries in Bangkok on Monday, escalating their monthlong antigovernment campaign and raising fears that the country could return to the unrest that followed the ouster in 2006 of Ms. Yingluck's brother, Thaksin...